A variety of geologic formations are often encountered as a wellbore is progressed during the drilling of an oil and gas well. Often these formations are comprised of very hard and sometimes brittle material such as rock, stone or shale. A rotating drill bit is placed at the end of a pipe string and is used to advance the wellbore through the various geologic formations. The rotary motion of the drill bit aids in removing the fragments of formation so that they may be carried uphole back to surface via circulating drilling mud pumped through the drillstring from the surface and exiting the drill bit.
A typical drill bit used for hard and rocky formations will have rotating cutters that are comprised of very hard materials such as PDC (polycrystalline diamond compact). These rotating cutters shear or scrape away rocky formation material as they are forcibly dragged along the surface of formation being drilled on each revolution. Drill bits having roller cone cutters work in a similar manner in that the teeth or cutters mounted on each cone will gouge, scrape, or chip away formation material as the bit advances.
The hard and brittle formations that are frequently encountered can be difficult to drill by merely rotating a drill bit. Drilling through such formation is often significantly improved by the addition of percussive motions or forces onto the drill bit. These percussive forces cause the rock to shatter and fragment as the rotating bit is advanced. This process is similar to the use of an ordinary electric drill when attempting to drill a hole in concrete (not a hammer drill). The rotation of the drill bit alone is ineffective as the rotation will cause the bit to simply heat up and become dull rather than advance the hole. When a hammer drill is utilized, the percussive motion of the hammer drill enhances the drilling progress and the drill bit lasts many times longer.
Many percussive devices on the market today rely on “weight on bit” to be applied to cause axial movement within the percussive device to initiate the percussive motion or hammering effect of the percussive device on the drill bit. If the weight on bit is too great, the percussive device stalls thus stopping the percussive effect. The driller must then pick up the drillstring and then set back down to restart the percussive device. This process is very time consuming, therefore is not an adequate or economical solution. Other percussive devices rely on turbine technology to power the percussive device. Such turbine technology is very complex and the percussive devices that rely on such technology are expensive and complicated to manufacture.
Another disadvantage to some percussive devices is that such devices have no mechanism for or method of increasing or decreasing the magnitude of percussive forces applied to the drill bit. Yet another disadvantage of many percussive devices is that these devices have relatively small impact areas or anvil surfaces. Impact forces on small contact surfaces areas produces high localized stresses which leads to excessive wear and ultimately premature failure of the percussive devices.
Further, in drilling and or workover operations, it is often necessary to pump a ball through the components of a BHA (bottom hole assembly) in order to shift sleeves, disconnect, or otherwise manipulate components of tools positioned on the drillstring downhole from the percussive device. It would be an advantage to provide a percussive device that allows for a ball to be pumped down the pipe string and through the device for manipulation of a tool positioned downhole from the percussive device. Similarly, it would be an advantage for a percussive device to allow for the passage of wireline tool strings through the percussive device for the manipulation of tool components positioned further downhole on the drillstring.
Consequently, there is a need for a hydraulic percussive apparatus that will serve to impart repeated impacts without the negative attributes noted above and with the aforesaid advantages.